Give David the magnum you've got, so he has a weapon, and tell David to be ready to run/shoot Dyunis if he does anything crazy... but don't like aim your weapons at him or anything, just be ready to in case. We'll soon get a judge of his character.

Careful about upsetting Dyunis' feelings, btw Blarg. He may be a lovable psychopath, but he could be dangerous if we get on his bad side, and as rovar has said before, death is entirely possible in this game.

Edited:

Speaking of which, where is that soda can and needle of liquid? We should go get them at some stage and fill them with vitae and give to David, so that he can revive us in case we die.

Creator's Note: Remember, this is all real time, I'm doing things step by step. You have choices, this is an example.
You can shoot Dyunis, then face the consequences, or just shout to stop, let him kill him, etc.

Tell Dynunis "You do understand you're not completely free. They injected something in you that will kill you within 1 hour of leaving stasis and only the antidote will save you. Kill Dave and you can forget about getting your pointless vendetta."